1. Field of the Invention
The field of this invention relates generally to devices and methods for fluid-solid and fluid-fluid mass transfer and in particular to an apparatus and a method for conducting mass transfer operations such as adsorption and ion-exchange using a bed of stratified porous granules in a chamber that has a variable cross-sectional area along the length of the chamber.
2. Description of the Prior Art
Mass transfer operations such as adsorption and ion-exchange involve the transfer of one or more components from a fluid mixture on to a solid granule that is relatively porous in nature. Adsorption and ion-exchange operations are used in chemical processing in bulk separation of chemicals or for purification in water and wastewater treatment to remove trace contaminants, in air pollution control for the removal of gaseous impurities, in medical applications for the purification of fluids, etc. In normal applications, large volumes of fluid are contacted with a small mass of the adsorbent in fixed-beds or fluidized-beds, and as the fluid passes through the bed, material is transferred from the fluid phase to the solid phase. Once the bed is partially or fully loaded, the adsorption step is terminated and the adsorbent is regenerated by a variety of methods. By operating a number of adsorbent beds simultaneously, columns can be cycled on and off from adsorption to regeneration steps, thereby providing continuous operation.
The large volume of fluid processed and the number of cycles imply that pumping energy to move the fluids across the bed is a significant cost factor. In addition, for mass transfer operations, including adsorption and ion-exchange, the process efficiency can be increased by increasing the rate at which mass is transferred from the fluid phase to the solid phase. It is desirable to strike a balance between increased mass transfer rate and low column pressure drop to increase the process efficiency.
In many applications, the fluid to be treated contains particulate matter that will get trapped in the fixed bed. Pressure drop will increase, due to the build up of particulate matter and hence the bed must be backwashed periodically. In some cases such as in the purification of fermentation broths, water and wastewater treatment, etc. microorganism growth may also occur, requiring frequent backwash operations. In conventional fixed beds using a range of adsorbent sizes, the particles will be fluidized during backwash and distribute themselves in such a manner that fine particles are at the top section of the bed, and coarse particles are at the bottom of the bed. If the fluid to be treated contains a large amount of particulate matter, the top section of the bed will clog rapidly while the bottom portion of the bed will be relatively free of particulates. This will increase the column pressure drop and decrease the overall efficiency of the mass transfer operations.